Receiving part for receiving a rod for coupling the rod to a bone anchoring element and a bone anchoring device with such a receiving part

ABSTRACT

A receiving part for receiving a rod for coupling the rod to a bone anchoring element includes a receiving part body with a top end, a bottom end, a central axis, a channel for receiving a rod, and an accommodation space for accommodating a head of a bone anchoring element, the accommodation space having an opening at the bottom end for introduction of the head, and a single piece pressure element having a top end, a bottom end, and a flexible portion to clamp the head, wherein the flexible portion has a first slit extending partially around the central axis when the pressure element is in the receiving part body, and a second slit extending from the bottom end of the pressure element to the first slit.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to and the benefit of U.S.Provisional Patent Application Ser. No. 61/652,646, filed May 29, 2012,the contents of which are hereby incorporated by reference in theirentirety, and claims priority from European Patent Application EP 12 169894.8, filed May 29, 2012, the contents of which are hereby incorporatedby reference in their entirety.

BACKGROUND

1. Field

The invention relates to a receiving part for receiving a rod forcoupling the rod to a bone anchoring element, wherein the receiving partincludes a receiving part body with a channel for receiving the rod andan accommodation space for accommodating a head of the bone anchoringelement and a pressure element with a flexible portion to clamp thehead. The flexible portion comprises a first slit provided in acircumferential direction with respect to a head of the bone anchoringelement and a second slit extending from an open end of the pressureelement into the first slit so that a slit ring is integrally formed inthe pressure element. Such a receiving part together with a multitude ofdifferent bone anchoring elements may constitute a modular polyaxialbone anchoring system.

2. Description of Related Art

Various designs of polyaxial bone screws, wherein the head is clampedfrom the side to lock the angular position of the bone screw, are known.

U.S. Pat. No. 5,672,176 describes a bone screw with a receiving partwith a conically shaped seat and a conically shaped pressure elementwhich exerts a pressure onto the head from above and from the side. Ifthe cone angle has a value laying within a specific range self-lockingof the pressure element within the receiving part takes place whichallows to preliminary lock the head within a receiving part while therod is still movable in order to allow the adjustment of its position.

U.S. Pat. No. 6,063,090 relates to a device used to connect alongitudinal support to a pedicle screw by an accommodating head havinga channel to accommodate the longitudinal support. The pedicle screw andthe accommodating head are connected via a conical collet chuck in theaccommodating head and by a spherical head on the pedicle screw. Thedevice allows engagement of the pedicle screw in the accommodating headafter the pedicle screw has been inserted into the bone.

US 2010/0234902 A1 describes a receiving part for receiving a rod forcoupling the rod to a bone anchoring element wherein the receiving partcomprises a receiving part body and a pressure element arranged therein.The pressure element is movable along a longitudinal axis of thereceiving part body from an inserting position insert the head, to apre-locking position to clamp the head in the receiving part by apre-stress exerted by the pressure element, to a locking position tolock the head in the receiving part.

SUMMARY

It is the object of the invention to provide an improved receiving partfor receiving a rod for coupling the rod to a bone anchoring element anda bone anchoring device with such a receiving part, which consists offew parts and has an improved handling during surgery.

The object is solved by a receiving part according to claim 1 and a boneanchoring device according to claim 17. Further developments are givenin the dependent claims.

The receiving part comprises a pressure element that has a slit ring atits bottom end. The slit ring is integrally connected to the pressureelement, which means the slit ring and the remainder of the pressureelement are a monolithic piece. The slit ring can expand in radialdirection to allow the insertion of the head of the bone anchor. Theforce necessary for introducing the head into the flexible portion ofthe pressure element is reduced compared to known pressure elements thathave only longitudinal, i.e. coaxial slits.

Furthermore, the distance that is necessary for moving the pressureelement in an axial direction until the expansion of the slit ringallows the insertion of the head of the bone anchor is reduced. Thisallows and facilitates an in situ mounting procedure of the receivingpart to a bone anchor that has already been inserted into the bone.

By means of the elasticity of the slit ring, the head of the boneanchoring element can be clamped by friction between the head and theslit ring. The friction fit between the head of the bone anchor and thepressure element allows a preliminary holding of the bone anchorrelative to the receiving part in a desired angular position of the boneanchor. This position can be changed by overcoming the friction force aslong as the bone anchor is not finally locked with respect to thereceiving part.

The receiving part and the bone anchor can be easily disassembled usinga simple tool.

Furthermore, the polyaxial bone anchoring device has only few parts thatreduces the costs for manufacturing and facilitates handling. Becausethe slit ring is integrally formed in the pressure element it can notget lost. The pressure element may be secured in the receiving part bodyagainst falling out.

The polyaxial bone anchoring device that includes the receiving partallows to provide a modular system wherein various bone anchors and/ordifferent receiving parts are available and can be assembled on demandprior to use.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the description of embodiments using the accompanying drawings. Inthe drawings:

FIG. 1: shows a perspective exploded view of a first embodiment of thebone anchoring device.

FIG. 2: shows a perspective view of the bone anchoring device of FIG. 1in an assembled state.

FIG. 3: shows a cross-sectional view of the bone anchoring deviceaccording to the first embodiment of FIG. 2, the section takenperpendicular to the rod axis.

FIG. 4: shows a perspective view of the pressure element.

FIG. 5: is a side view of the pressure element.

FIG. 6: shows a top view of the pressure element.

FIG. 7: shows a cross-sectional view of the pressure element, thesection taken along line A-A of FIG. 6.

FIG. 8: shows a bottom view of the pressure element.

FIGS. 9 to 14: show steps of assembling the bone anchoring element andthe receiving part of the bone anchoring device according to the firstembodiment.

FIGS. 15 and 16: show two positions of the pressure element in thereceiving part with inserted bone anchoring element.

FIG. 17: shows a step of disassembling the receiving part and the boneanchor using a tool.

FIG. 18: shows a perspective exploded view of a bone anchoring deviceaccording to a second embodiment.

FIG. 19: shows a perspective view of the pressure element of the boneanchoring device according to the second embodiment.

FIG. 20: shows a side view of the pressure element according to thesecond embodiment.

FIG. 21: shows a cross-sectional view of the pressure element accordingto FIG. 19.

FIGS. 22 and 23: show steps of assembling and locking with respect tothe bone anchoring device according to the second embodiment.

FIG. 24: shows a perspective view of a further embodiment of thepressure element of the bone anchoring device.

FIG. 25: shows the pressure element shown in FIG. 24 from a bottomperspective view.

FIG. 26: shows a side view of the pressure element of FIG. 24.

FIG. 27: shows a top view of the pressure element of FIG. 24.

FIG. 28: shows a bottom view of the pressure element of FIG. 24.

FIG. 29: shows a cross-sectional view of the pressure element of FIG. 24along line D-D shown in FIG. 27.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 2, a bone anchoring device according to a firstembodiment comprises a bone anchoring element 1 in the form of a bonescrew having a threaded shaft 2 and a head 3. The head 3 has a sphericalouter surface portion including the greatest outer diameter E of thesphere and a flat free end with a recess 4 for engagement with ascrewing-in tool. The bone anchoring device further comprises areceiving part for receiving a stabilization rod 100 and for couplingthe stabilization rod 100 to the bone anchoring element 1. The receivingpart consists of a receiving part body 5 and a pressure element 6arranged in the receiving part body 5. The pressure element 6 serves forlocking the head 3 in the receiving part body 5. Pins 8 may be providedfor holding the pressure element 6 in the receiving part body 5.

Further, a closure element 7 in the form of an inner screw is providedfor securing the rod 100 in the receiving part body 5.

The receiving part body 5 will now be explained with reference to FIGS.1 to 3. It comprises a top end 10 and an opposite bottom end 11 and anaxis of symmetry M passing through the top end and the bottom end. Abore 12 is provided that is coaxial to the axis of symmetry M. In afirst region adjacent the top end 10, the receiving part body 5 has aU-shaped recess 13 with a bottom directed towards the bottom end 11 andtwo free lateral legs 14 a, 14 b extending towards the top end 10. Inthe region of legs 14 a, 14 b an internal thread 15 is provided thatcooperates with the inner screw 7. The channel formed by the U-shapedrecess 13 is sized so as to receive the rod 100 therein that connects aplurality of bone anchoring devices. In the region of the legs 14 a, 14b, a first portion of the bore 12 comprises a first inner diameter. In aregion below the legs 14 a, 14 b the bore 12 has a widened portion 12 awith a diameter greater than the diameter of the first portion. Betweenthe bottom end 11 and the widened portion 12 a, the bore 12 comprises anarrowing portion 12 b that tapers towards the bottom end 11 with a coneangle. An opening 16 is provided at the bottom end 11, the diameter ofthe opening 16 being larger than the diameter of the head 3 to allow theintroduction of the head 3 from the bottom end 11 of the receiving part.The widened portion 12 a and the narrowing portion 12 b provide anaccommodation space 17 for the head 3 of the bone anchor.

On each leg 14 a, 14 b, bores 19 a, 19 b extending through the legs in adirection substantially perpendicular to the central axis M are providedfor receiving the pins 8 a, 8 b. The bores 19 a, 19 b are locatedapproximately at a center of each leg. The pins 8 a, 8 b have preferablysuch a length that once inserted into the bores 19 a, 19 b, they extenda short distance into the bore 12 to provide a stop for the pressureelement 6 described below. The pins 8 a, 8 b may be flush with the outersurface of the receiving part body 5.

As can be seen in particular in FIGS. 1 and 4 to 8, the pressure element6 has a top end 61 and an opposite bottom end 62. Adjacent the top end61 there is a substantially cylindrical portion 63 which has an outerdiameter that is slightly smaller than the inner diameter of the bore12, so that the pressure element 6 is moveable in the bore 12. There isa U-shaped recess 64 that is open to the top end 61 for receiving therod 100 therein when the pressure element 6 is arranged in the receivingpart body 5 such that the U-shaped recess 64 is aligned with theU-shaped recess 13 of the receiving part body 5. Adjacent thecylindrical portion 63, a recessed portion 65 is provided that continuesinto a head receiving portion 66.

The head receiving portion 66 comprises a hollow interior chamber 67that is substantially spherically-shaped and configured to accommodatethe spherical head 3 therein. At the bottom end 62, the head receivingportion 66 has an opening to allow the introduction of the head 3. In aregion adjacent the recessed portion 65, the head receiving portion 66has a spherical outer surface portion that continues into a taperedouter surface portion in the direction towards the second end 62. Thetaper corresponds substantially to the paper of the narrowing portion 12b of the receiving part.

At a distance from the bottom end 62 a circumferentially extending slit70 is provided. The slit 70 is substantially perpendicular to thecentral axis M of the receiving part body when the pressure element isarranged in the receiving part body 5. The slit 70 extends to aroundmore than 180°, preferably more than 270° and further preferably morethan 340° around the central axis M. Hence, by means of the slit 70, aring-shaped portion 71 at the bottom end 62 is provided that isintegrally connected to the pressure element as a whole by a wallportion forming a connecting strip 72.

The connecting strip 72 has such a length in a circumferential directionthat it provides a stable connection of the ring-shaped portion 71 tothe pressure element 6. At one end of the circumferentially extendingslit 70, there is a coaxially extending slit 73 that extends from thesecond end 62 fully through the ring-shaped portion 71 into the slit 70.By means of this, the ring-shaped portion is cut through incircumferential direction and forms a slit ring 71 that can be expandedand compressed in the radial direction. The width of the vertical slit73 is preferably smaller than the width of the circumferential slit 70.This can be seen in particular in FIGS. 3 and 7. The outer surface ofthe slit ring 71 is tapered towards the bottom end 62 of the pressureelement 6. The connecting strip 72 and the vertical slit 73 are arrangedat an angle relative to the U-shaped recess 64. The position and size ofthe slit ring 71 is such that when the head 3 of the bone anchoringelement is inserted from the open bottom end 62 of the pressure element,the slit ring 71 expands in that the width of the vertical slit becomeslarger, and when the head 3 has been fully inserted into the hollowinterior chamber 67, the slit ring encompasses the head 3 at or belowthe greatest diameter E of the head 3 in the direction to the shaft 2.

The maximum outer diameter of the head receiving portion 66 is slightlysmaller than the inner diameter of the bore 12 in the first portion andis smaller than the diameter of the bore in the widened portion 12 a.Hence, the slit ring 71 is configured to expand in the widened portion12 a of the receiving part body 5.

At the lateral sides of the U-shaped recess, the pressure elementcomprises two elongate recesses 69 a, 69 h that are elongate in thedirection from the top end to the bottom end. The recesses 69 a, 69 bare provided in the outer wall. These elongate recesses 69 a, 69 b servefor engagement by the pins 8 a, 8 b, respectively, when the pressureelement is inserted and provide for a stop when the pins abuts againstone of the ends of the recesses in axial direction.

Finally, the pressure element 6 comprises a coaxial through hole 75allowing access to the recess 4 of the head 3 of the hone anchor with atool (not shown). The through hole 75 is preferably provided in at leasta portion thereof with an internal thread as shown in FIGS. 3 and 7. Theinternal thread serves for engagement with a tool to fix the pressureelement when head 3 of the bone anchor shall be removed from the headreceiving portion 66 to be described below.

The pressure element 6 may be arranged in the receiving part byintroduction from the top end 10 of the receiving part and by aligningthe U-shaped recesses of the pressure element and the receiving partbody. The pins 8 a, 8 b may be inserted thereafter to extend into theelongate recesses 69 a, 69 b of the pressure element to hold thepressure element within the receiving part thereby preventing escapingof the pressure element through the first end 10. By means of this thepressure element and the receiving part body are pre-assembled and canbe connected to a bone anchoring element.

All parts described before may be made of a bio-compatible material,such as a bio-compatible metal like stainless steel or titanium, or abio-compatible metal alloy, e.g., Nitinol, or are made from abio-compatible plastic material, e.g. Polyetheretherketon (PEEK). Theparts may be all of the same material or of different materials.

Steps of assembling the bone anchoring device will now be explained withreference to FIGS. 9 to 14. As shown in FIG. 9, the pressure element andthe receiving part body 5 are pre-assembled as described above. The slitring 71 abuts against the upper part of the narrowing portion 12 b. Thebone anchoring element 1 is introduced from the bottom end 11 of thereceiving part body (arrow A). Next, as shown in FIG. 10, the head 3 ofthe bone anchoring element is introduced from the bottom end 11 andtouches the slit ring 71 of the pressure element 6, thereby shifting thepressure element in the direction of the arrow B until the lower end ofthe elongate recesses 69 a, 69 b abut against the pins 8 a, 8 b,respectively, as shown in FIG. 11. In this portion, the slit ring 71 issituated in the accommodation space provided by the widened portion 12 aof the receiving part body 5.

Further introduction of the head 3 into the head receiving portion 66expands the slit ring 71 within the widened portion 12 a of thereceiving part body 5. The head 3 can be inserted. Because the slit ring71 does not expand at the connecting strip 72, the introduction of thehead 3 is not precisely coaxially to the axis of symmetry M but isslightly out of the axis of symmetry M. By the further introduction ofthe head 3 the slit ring 71 is expanded to the maximum extent as shownin FIG. 13. Still further introduction of the head 3 allows the head 3to enter the upper space of the hollow interior chamber 67 until itabuts against the inner wall of the chamber 67. Thereby the slit ring 71elastically contracts around the head 3 as shown in FIG. 14.

When the head 3 abuts against the inner wall, of the head receivingportion 66 in the upper region, the slit ring 71 encompasses the portionof the head below the equator E as can be seen in FIG. 15. In thiscondition, the slit ring 71 frictionally clamps the head 3 so that thebone anchoring element can be pivoted to a desired angular position andheld there by the friction fit between the pressure element 6 and thehead 3.

Finally, moving down the pressure element 6 presses the slit ring 71into the narrowing portion 12 b so that the slit ring 71 locks the headas shown in FIG. 16.

In clinical use, first the receiving part may be assembled with the boneanchoring element as described above, then the bone anchoring elementmay be inserted into the bone. Usually at least two bone anchoringdevices are inserted into the bone and the receiving parts are aligned.Then, the rod is inserted into the receiving part 9. Finally, the innerscrew 7 is screwed between the legs 14 a, 14 b of the device and the rodwith the pressure element is pressed down so that the head 3 is locked.

Alternatively, the bone anchoring element can be inserted first into thebone and the receiving part body 5 with the pressure element 6 can bemounted thereafter onto the head 3 of the bone anchoring element.Because of the slit ring 71 the force necessary to insert the head intothe head receiving portion 66 of the pressure element is reduced and thedistance needed to move the pressure element within the bore 12 of thereceiving part body is reduced.

In the case that the receiving part body 5 and the head 3 shall bedisassembled, i.e. the head 3 shall be removed from the receiving partbody 5, a tool 200 may be used as shown in FIG. 17. The tool 200 mayhave a first portion to be gripped by a person (not shown) and athreaded end portion 201 that engages the threaded through hole 75 ofthe pressure element 6. By means of this, the pressure element 6 can bepulled into the direction of the top end 10 of the receiving part bodyuntil the pins 8 a, 8 b abut against the lower end of the elongaterecesses 69 a, 69 b. In this position, the slit ring 71 is locatedwithin the accommodation space in the widened portion 12 a of thereceiving part body. Then, the head 3 can be removed through the bottomend 11 of the receiving part body 5.

A second embodiment of the bone anchoring device is shown in FIGS. 18 to23. With reference to FIG. 18, the bone anchoring device according tothe second embodiment differs from the bone anchoring device accordingto the first embodiment in that the receiving part body 5′ has only onelateral bore 690 provided in the leg 14 a at a distance below the bottomof the U-shaped recess 13. Hence, as shown in FIGS. 22 and 23, a singlepin 80 extends through the bore 690 into the widened portion 12 a of thebore 12. All other parts of the receiving part body are identical to thefirst embodiment and the description thereof will not be repeated.

The pressure element 6′ comprises at the end of the cylindrical portion63 a first substantially semicircular notch 630 a that is open towardsthe bottom end 62 and configured to engage with the pin 80 as shown inFIG. 23. Hence, the notch 630 a is provided at 90° to the U-shapedrecess 64.

The pressure element 6′ further comprises a second notch 670 a that isprovided in an axial direction below the first notch 630 a in the upperpart of the head receiving portion 66. The width of the second notch 670a is slightly larger than the diameter of the pin 80 so that the pin 80can engage the second notch 670 a. As can be seen in particular in FIG.21, the second notch 670 a has such a depth that it is recessed withrespect to the slit ring 71. The notches 630 a, 670 a are arranged onthe opposite side of the connection strip 72, as best seen in FIGS. 20and 21.

In use, the pressure element 6′ and the receiving part body 5′ arepre-assembled. The head 3 is introduced like in the first embodimentfrom the bottom end 11. When the head 3 is introduced and abuts againstthe bottom end of the receiving part, the slit ring 71 abuts against thepin 80 as shown in FIG. 22. Therefore, the pin 80 provides a stop forthe slit ring 71 and prevents bending of the slit ring in a directiontowards the top end 10 of the receiving part body 5. Simultaneously, thepin 80 acts as a securing device against escaping of the pressureelement 6′ and as positioning device for a correct alignment of thepressure element 6′ with respect to the receiving part body 5′.

A further embodiment of the pressure element 6″ will be described withreference to FIGS. 24 to 29. The pressure element 6″ has as in theprevious embodiments, a top end 61 and a bottom end 62. Adjacent the topend 61, a substantially cylindrical portion 63′ is provided that extendsto a distance from the bottom end 62 and comprises adjacent the top end61 the U-shaped recess 64. A central bore 75 with a threaded portion ispresent as in the previous embodiments.

At a distance from the bottom end 62 two slits 700, 701 are providedthat extend in a transverse direction relative to the central axis M. Atthe end of each of the horizontal slits 700, 701 a vertical slit 703,704 is provided, wherein the vertical slits are located at thecircumference of the pressure element on opposite places. The verticalslits 703, 704 extend from the bottom end 62 fully into the horizontalslits 700, 703, respectively. Hence, two ring portions 710 a, 710 b thatare opposite to each form the flexible section of the pressure element.The ring portions 710 a, 710 b are connected with the cylindricalportion 63′ via connecting strips 702 a, 702 b located opposite to eachother. By means of the two ring portions 710 a, 710 b a slit ring 71′ isformed. The slit ring 71′ can expand, wherein the vertical slits 703,704 are enlarged and can elastically return to its non-expandedcondition.

Because the pressure element is almost not flexible in the region of theconnecting strips 702 a, 702 b a recess 705 may be provided on one orboth of the inner wall portions of the connecting strips 702 a, 702 bsuch that the inner diameter of the pressure element in the region ofthe connecting strips 702 a, 702 b is the same or slightly larger thanthe largest outer diameter E of the head 3.

Further, the pressure element comprises the interior chamber 67 with apreferably spherically-shaped section to accommodate the head 3.

The pressure element 63′ can be combined with a receiving part bodyaccording to the first and second embodiment. The features of theembodiments can be combined among each other. The pressure element canbe further modified, for example, more than two horizontal and twovertical slits may be provided.

Further modifications of the embodiment shown are possible. For example,the circumferential slit 70, 700, 701 needs not to be perpendicular tothe central axis, it can also be inclined. The vertical slit 73, 703,704 can also be inclined or can have a particular contour, as long as itextends fully through the ring. The head of the bone anchoring elementcan have any other shape, e.g., a cylindrical shape wherein a monoaxialbone screw is provided allowing rotation of the screw element withrespect to the receiving part around a single axis. The head 3 can alsobe, e.g., conically shaped. The hollow interior chamber is then adaptedto the shape of the head. In a further modification, the receiving partbody can have an inclined bottom end to allow a greater angulation ofthe head in one direction.

In another modification, the narrowing portion 12 d of the receivingpart body 5 in the corresponding outer surface portion of the slit ring71,71′ are not tapered but spherically curved.

For the bone anchoring element, all kinds of bone anchoring elements canbe used, such as screws of different length, of different diameter, ofdifferent threadforms, nails with barbs or without barbs, cannulatedscrews or nails, or hooks.

For the closure element all known kinds of closure elements can be used,single part closure elements, double part closure elements etc.

1. A receiving part for receiving a rod for coupling the rod to a boneanchoring element, the receiving part including a receiving part bodywith a top end, a bottom end and a central axis extending through thetop end and the bottom end; a channel for receiving the rod, and anaccommodation space for accommodating a head of the bone anchoringelement, the accommodation space having an opening at the bottom end forintroduction of the head; and a single piece pressure element located atleast partly in the accommodation space, the pressure element having atop end facing the top end of the receiving part and a bottom end facingthe bottom end of the receiving part and a flexible portion to clamp thehead; wherein the flexible portion comprises at least one first slit ata distance from the bottom end of the pressure element, the first slitextending partially around the central axis when the pressure element isin the receiving part body and a at least one second slit extending fromthe bottom end of the pressure element into the first slit.
 2. Thereceiving part of claim 1, wherein by the at least one first slit andthe at least one second slit a slit ring is formed.
 3. The receivingpart of claim 2, wherein the slit ring is connected to the pressureelement via at least one connecting strip in a monolithic manner and isconfigured to laterally encompass the head.
 4. The receiving part ofclaim 2, wherein the slit ring has an exterior tapered surface portionthat tapers towards the bottom end of the pressure element.
 5. Thereceiving part of claim 1, wherein the at least one second slit extendssubstantially coaxially to the central axis.
 6. The receiving part ofclaim 1, wherein the at least one first slit is substantially transverseto the central axis and preferably perpendicular to the central axis. 7.The receiving part of claim 1, wherein a single first slit and a singlesecond slit are provided.
 8. The receiving part of claim 1, wherein twofirst slits are provided and wherein two second slits are provided thatare preferable arranged opposite to each other.
 9. The receiving part ofclaim 1, wherein the at least one second slit is arranged at one end ofthe at least one first slit in a circumferential direction.
 10. Thereceiving part of claim 1, wherein the flexible portion of the pressureelement has a hollow spherically-shaped inner surface portion configuredto accommodate the head.
 11. The receiving part of claim 1, wherein theflexible portion is sized relative to the head such that the head isclamped by friction.
 12. The receiving part of claim 1, wherein thepressure element further comprises a substantially cylindrical portionwith a recess at its top end that is configured to accommodate the rod.13. The receiving part of claim 1, wherein a stop is provided thatprevents movement of the pressure element once it is arranged in thereceiving part towards the top end of the receiving part.
 14. Thereceiving part of claim 2, wherein the pressure element can assume inthe receiving part body an inserting position in which the slit ring isallowed to expand laterally in the space so as to allow introduction ofthe head through the opening at the bottom end.
 15. The receiving partof claim 1, wherein the accommodation space is substantially cylindricalwith a narrowing portion near the bottom end, the narrowing portionbeing preferably tapered or rounded.
 16. The receiving part of claim 15,wherein the pressure element comprises a narrowing portion at its outerwall which is preferably tapered or rounded and which cooperates withthe narrowing portion in the accommodation space.
 17. A bone anchoringdevice comprising a receiving part according to claim 1 and a boneanchoring element having a shaft and a head that preferably has aspherically-shaped outer surface portion and wherein preferably aclosure element is provided for securing the rod in the receiving part.